Electrodeposition, Characterization, Physicochemical Properties and Application of Lead Dioxide Coatings

Abstract

Lead dioxide (PbO₂) coatings are materials of great interest because of their unique combination of low cost, ease of preparation, high conductivity, high oxygen overpotential, good chemical stability in corrosive media and strong catalytic ability towards organic pollutants. However, there are still main issues to be addressed for the wider application of PbO₂ electrodes: (i) relatively low current densities and efficiencies for particular electrode reactions when applied in practical; (ii) long term stability, which is always a critical issue because corrosion of the coating will lead to contamination of the products with toxic Pb2+ ions. The ultimate objective of this research was to improve the electrode's efficiency and stability. To achieve this, a few strategies were taken, such as, optimisation of substrate pre-treatments, optimisation of the deposition processing parameters, incorporation of nanoparticles and doping with foreign ions on the electro-crystallization, in order to improve physicochemical properties and electrochemical performance of the PbO₂ coatings. The knowledge originated from this study would shed light on a further understanding of electrocrystallisation of PbO₂ and also provide a perspective on further possible improvement of the electrochemical performance of PbO₂ coatings in terms of service lifetime and electrocatalytic activity. PbO₂ coatings were electroplated on Ti substrates pre-coated with a Sb doped SnO₂ interlayer from a traditional acidic nitrate solution. It is found that an interlayer of SnO₂-Sb on Ti substrate can significantly promote the electrodeposition of PbO₂ coatings. By varying electrodeposition parameters including bath temperature, deposition time, current density and Pb²⁺ ion concentration, PbO₂ coatings with various morphologies, microstructures and crystallite sizes were synthesized. A preferential crystallographic orientation was usually observed in the coatings obtained at low temperature and the coating exhibited a caterpillar-like morphology. At a higher deposition temperature a more tightly packed pyramid-like morphology was observed. The crystallite size increased with deposition temperature. In most cases, a mixture of a + b-PbO₂ phase was obtained except at low current density. At a current density < 40 mA/cm², the coating exhibited a pure b-PbO₂ phase with a (301) texture and pebble-shaped morphology. In addition, the Pb²⁺ concentration appeared to have insignificant effect on the morphology and crystallographic orientation of the PbO₂ coatings. At a current density >= 40 mA/cm², the PbO₂ deposit exhibited a mixture of a + b-PbO₂ phase (a dominant b-PbO₂ phase with a minor a-PbO₂ phase present). Moreover, the morphology of the deposits is sensitive to the corresponding current density and Pb²⁺ concentration. It is indicated that either a sufficiently high concentration or low current density is necessary in order to obtain a compact and dense PbO₂ deposit. Otherwise non-uniform or porous deposits would result from severe concentration polarization at the electrode surface. It is concluded that electrochemical performance of the deposited PbO2 largely depends on their morphologies and microstructures. High surface area, high porosity and good connectivity between crystallites play a synergic role in determining the electrochemical performance of the PbO₂ coatings. Composite PbO₂ electrodes were prepared by a co-deposition method from Pb²⁺ plating solution containing either suspended nano-TiO₂ particles or dissolved bismuth ions. It is found that the incorporation of nano-TiO₂ particles significantly enhances the electrode's electrochemical stability. The service lifetime of the nanocomposite electrodes at the doping content of 0.8 g/L was almost three times longer than that of the undoped PbO₂ electrodes. The electrochemical activity of the nanocomposite electrodes towards benzoic acid (BA) degradation was dramatically promoted, which may be attributed to the much higher overpotential for oxygen evolution and more active sites on the nanocomposite electrodes. Meanwhile, the doping of bismuth into PbO₂ coating was investigated and different doping contents were employed. SEM images reveal that Bi-PbO₂ coatings present a compact structure with rod-shaped deposits and X-ray diffractograms demonstrated that incorporation of bismuth did not have significant effect on the structure of PbO₂ coatings. The oxygen overpotential on Bi-PbO₂ electrodes is significantly higher than on undoped PbO₂ electrode. The as-prepared Bi-PbO₂ electrodes were employed as anodes for electrolysis of target compound and the oxidation process was monitored by high-performance liquid chromatogram (HPLC). The higher electrocatalytic ability at Bi-PbO₂ can be attributed to the increased specific area of the electrodes resulted from the decrease in the size of the crystal particles, and can also be attributed to the favourable adsorption of ·OH radicals at Bi(V) sites.